1. Field of the Invention
The present invention relates to an organic electroluminescence cell excellent in light-emitting efficiency and especially excellent in efficiency of extracting luminescence to the outside, a (polarizing-type) planar light source of high efficiency using the organic electroluminescence cell, and a (liquid crystal) display device having the planar light source.
2. Description of the Related Art
An electroluminescence cell or a light-emitting diode in which a light-emitting layer is provided between electrodes to obtain luminescence electrically has been researched and developed actively not only for application to a display device but also for application to various types of light sources such as a flat illuminator, a light source for optical fiber, a backlight unit for liquid crystal display, a backlight unit for liquid crystal projector, etc.
Particularly, an organic electroluminescence cell has attracted public attention in recent years because it is excellent in light-emitting efficiency, low-voltage drive, lightweight and low cost. A primary concern with the purpose of application to these light sources is enhancement in light-emitting efficiency. Improvement in cell structure, material, drive method, production method, etc. has been examined to obtain light-emitting efficiency equivalent to that of a fluorescent lamp.
In an inter-solid luminescent element such as an organic electroluminescence cell in which luminescence is extracted from a light-emitting layer per se, however, light generated at a angle not lower than a critical angle decided on the basis of the refractive index of the light-emitting layer and the refractive index of an output medium is totally reflected and confined in the inside, so that the light is lost as guided light.
According to calculation based on classical law of refraction (Snell's law), light-extracting efficiency η in taking out generated light to the outside can be given by the approximate expression η=1/(2n2) in which n is the refractive index of the light-emitting layer. Assuming that the refractive index of the light-emitting layer is 1.7, then 80% or more of the light is lost as guided light, that is, as a loss in a side face direction of the cell because η is nearly equal to 17%.
In the organic electroluminescence cell, excitons contributing to luminescence are only singlet excitons among excitons generated by recombination of electrons and holes injected from the electrodes. The probability that singlet excitons will be generated is ¼. Even in the case where only such a thing is taken into consideration, the efficiency is not higher than 5%, that is, the efficiency is very low.
As a method for improving the light-emitting efficiency of the light-emitting layer per se, development of a luminescent material (Unexamined Japanese Patent Publication No. 2001-313178) for generating light also from phosphorescence due to triplet excitons has advanced in recent years, so that the possibility that quantum efficiency will be improved remarkably has been found.
Even if quantum efficiency were improved, light-emitting efficiency would be reduced in accordance with light-extracting efficiency multiplied by the quantum efficiency. In other words, if light-extracting efficiency can be improved, there is room for remarkable improvement in light-emitting efficiency according to the synergy between the quantum efficiency and the light-extracting efficiency.
As described above, in order to extract guided light to the outside, a region for disturbing an angle of reflection/refraction need to be formed between the light-emitting layer and an emergence surface to destroy Snell's law to thereby change an angle of transmission of light originally totally reflected as guided light or beam-condensing characteristic need to be given to luminescence per se. It is however not easy to form such a region that outputs all guided light to the outside. Therefore, a proposal for taking out guided light as much as possible has been made.
For example, as methods for improving light-extracting efficiency, there have been proposed a method in which beam-condensing characteristic is given to a substrate per se to improve light-extracting efficiency (Unexamined Japanese Patent Publication No. Sho. 63-314795), a method in which a light-emitting layer is made of discotic liquid crystal to improve frontal directivity of generated light per se (Unexamined Japanese Patent Publication No. Hei. 10-321371) and a method in which a stereostructure, an inclined surface, a diffraction grating, etc. are formed in the cell per se (Unexamined Japanese Patent Publication No. Hei. 11-214162, Unexamined Japanese Patent Publication No. Hei. 11-214163 and Unexamined Japanese Patent Publication No. Hei. 11-283751). These proposals, however, have a problem on complication in structure, reduction in light-emitting efficiency of the light-emitting layer per se, etc.
As a relatively simple method, there has been also proposed a method in which a light-diffusing layer is formed to change an angle of refraction of light to thereby reduce light satisfying the condition of total-reflection.
For example, there have been proposed various methods such as a method using a diffusing plate having a transparent substrate, and particles-dispersed in the transparent substrate so as to form such a distributed index structure that the refractive index of the inside is different from the refractive index of the outside (Unexamined Japanese Patent Publication No. Hei. 6-347617), a method in which a diffusing member having a light-transmissive substrate, and a single particle layer arranged on the light-transmissive substrate (Unexamined Japanese Patent Publication No. 2001-356207) are used, and a method in which scattering particles are dispersed in the same material as that of the light-emitting layer (Unexamined Japanese Patent Publication No. Hei.6-151061). These proposals have been provided by finding features in the characteristic of scattering particles, the refractive index difference from a dispersion matrix, the dispersing form of particles, the place for formation of the scattering layer, and so on.
Incidentally, the organic electroluminescence cell uses such a principle that holes injected from the anode and electrons injected from the cathode by application of an electric field are recombined into excitons to generate luminescence from a fluorescent (or phosphorescent) substance. It is therefore necessary to perform the recombination efficiently in order to improve quantum efficiency. As this method, there is generally used a method in which the cell is formed as a laminated structure. For example, a two-layer structure having a hole transport layer and an electron transport light-emitting layer or a three-layer structure having a hole transport layer, a light-emitting layer and an electron transport layer is used as the laminated structure. There have been also made various proposals for a laminated cell formed as a double hetero structure in order to improve efficiency.
In such a laminated structure, recombination is substantially concentrated in a certain region. For example, in a two-layer type organic electroluminescence cell, as shown in FIG. 12, recombination is concentrated in an electron transport light-emitting layer side region 6 which is about 10 nm distant from an interfacial layer between a hole transport layer 4 and an electron transport light-emitting layer 5 which are sandwiched between a pair of electrodes constituted by a reflective electrode 3 and a transparent electrode 2 (as reported by Takuya, Ogawa et al, “IEICE TRANS ELECTRON” Vol. E85-C, No. 6, p. 1239, 2002).
Light generated in the light-emitting region 6 is radiated in all directions. Consequently, as shown in FIG. 13, an optical path difference is produced between light radiated toward a light-extracting surface on the transparent electrode 2 side and light radiated toward the reflective electrode 3, reflected by the reflective electrode 3 and radiated toward the light-extracting surface.
In FIG. 13, the thickness of the electron transport light-emitting layer in the organic electroluminescence cell is generally in a range of from tens of nm to a hundred and tens of nm, that is, the order of wavelength of visible light. Accordingly, light beams finally outgoing from the cell interfere with each other. The interference becomes destructive or constructive according to the distance d between the light-emitting region and the reflective electrode. Although only light radiated in a frontal direction is shown in FIG. 13, light radiated in oblique directions is also present actually. The condition of interference varies according to the angle of radiated light in addition to the distance d and the wavelength λ of generated light. As a result, there may occur the case where light beams radiated in a frontal direction interfere with each other constructively but light beams radiated in a wide-angle direction interfere with each other destructively, or there may occur the case reverse to the aforementioned case. That is, luminance of generated light varies according to the viewing angle.
It is a matter of course that the intensity of light varies remarkably according to the angle as the distance d increases. Therefore, the thickness of the electron transport light-emitting layer is generally selected so that the distance d is made equal to about a quarter of the wavelength of generated light to obtain constructive interference of light in the frontal direction.
When, for example, the distance d is smaller than about 50 nm, absorption of light becomes remarkable in the reflective electrode generally made of a metal. This causes reduction in intensity of generated light and influence on intensity distribution. That is, in the organic electroluminescence cell, the distribution of radiated light varies remarkably according to the distance d between the light-emitting region and the reflective electrode, so that the guided light component varies widely according to the variation in the distribution of radiated light. Furthermore, the emission spectrum of this type cell has broad characteristic in a relatively wide wavelength range. Accordingly, variation in the wavelength range for constructive interference of light according to the distance d causes variation in peak wavelength of generated light. Furthermore, the emission spectrum varies according to the viewing angle in addition to the distance d.
To solve these problems, there has been made a proposal for selecting the film thickness to suppress a phenomenon that the color of generated light varies according to the viewing angle (see Patent Document 1). In this proposal, however, there is no description concerning guided light. It is obvious that the film thickness range selected by this proposal for suppressing the dependence of the color of generated light on the viewing angle is different from the range according to the invention which will be described later.
For the aforementioned reason, the light-extracting efficiency of the laminated organic electroluminescence cell cannot be calculated correctly on the classical assumption that about 80% of generated light is confined as guided light in the inside of the cell. That is, the guided light component varies remarkably according to the structure of the cell. For example, as reported by M. H. Lu et al. (J. Appl. Phys., Vol. 91, No. 2, p. 595, 2002), detailed research on change in the guided light component according to the structure of the cell has been made on the basis of a quantum-mechanical calculation method in consideration of a micro-cavity effect.
Accordingly, there is a possibility that the obtained effect will not be so large as estimated by the classical theory even in the case where a light-diffusing layer or the like is formed in order to destroy the condition of total reflection.
When the organic electroluminescence cell is used as a backlight unit for a liquid crystal display device, luminescence generated from the cell needs to be converted into linearly polarized light by a polarizer when used for liquid crystal display because the luminescence is natural light. As a result, absorption loss due to the polarizer is produced. There is a problem that the rate of utilization of light cannot be set to be higher than 50%. Hence, even in the case where guided light is extracted efficiently by the aforementioned method, a half or more of the guided light is absorbed to the polarizer.
As a method for solving the problem, there has been made a proposal for forming an organic electroluminescence cell layer on an oriented film to extract luminescence per se as linearly polarized light (see Patent Document 2). Although the absorption loss due to the polarizer can be reduced to half, at the most, by the aforementioned proposal, there is a possibility that the light-emitting efficiency of the cell will be lowered because of the insertion of the oriented film for orienting an organic thin film. In addition, like the related-art cell, the problem of the guided light due to total reflection cannot be solved at all by this proposal.
It has been proposed a method in which light generated in an organic electroluminescence cell is extracted through a polarizing/scattering film (see Patent Document 3). According to this proposal, light lost as guided light can be scattered so as to be extracted, and output light can be extracted as polarized light which is rich in linearly polarized light. Accordingly, the absorption loss due to the polarizer can be reduced, so that a polarizing-type planar light source of high efficiency can be provided as a light source for a liquid crystal display device.
For example, the relation between the guided light and the influence of the distance between the light-emitting region and the reflective electrode on interference has not been described yet in this proposal. It cannot be said that this proposal brings out the greatest possible effect of the light source for a liquid crystal display device.
As a method for reducing absorption loss of backlight due to a polarizer in a liquid crystal display device, there is known a method using a polarized light separating layer made of a reflection type polarizing element (see Patent Documents 4 and 5). There has been made a proposal for applying this method to an organic electroluminescence cell (see Patent Documents 6 and 7).
No proposal has ever been made for bringing out the greatest light-emitting efficiency, for example, by combination of an organic electroluminescence cell and a reflection type circular polarizing element on the assumption of detailed research on the relation between the guided light and the influence of the distance between the light-emitting region and the reflective electrode on interference in the organic electroluminescence cell. Therefore, the provision of a polarizing-type planar light source of high efficiency best adapted to a liquid crystal display device using polarized light is desired earnestly in the existing circumstances.
[Patent Document 1]
Unexamined Japanese Patent Publication No. Hei. 5-3081 (pages 2 to 4)
[Patent Document 2]
Unexamined Japanese Patent Publication No. Hei. 11-316376 (pages 2 to 5)
[Patent Document 3]
Unexamined Japanese Patent Publication No. 2001-203074 (pages 2 to 6)
[Patent Document 4]
Unexamined Japanese Patent Publication No. Hei. 4-268505 (pages 2 to 6)
[Patent Document 5]
Unexamined Japanese Patent Publication No. Hei. 8-271892 (pages 2 to 5)
[Patent Document 6]
Unexamined Japanese Patent Publication No. 2001-244080 (pages 2 to 4)
[Patent Document 7]
Unexamined Japanese Patent Publication No. 2001-311826 (pages 2 and 3)